Laser vibrometer for vibration measurements

ABSTRACT

A laser diode (1) of a laser vibrometer emits light which is guided onto a polarization beam splitter (3) by a lens 2. A portion of the beam is guided through directly to an imaging lens (4) and a second portion is deflected onto a mirror (5). Quarter-wave plates (6) and (7) are arranged between the polarization beam splitter and respectively the lens or the mirror. The beam which is reflected on the mirror is guided completely through the polarization beam splitter onto a photodetector unit (8). The beam reflected by the object (9) to be measured, which swings in the direction (14), is also deflected to the photodetector unit. The output signal of the photodetector unit, the interference signal generated from the superposition of the two reflected beams, is supplied to an electronic evaluation equipment (10). The latter is also supplied with a signal of a current modulator (11) which, on the other hand, supplies the laser diode with modulated injection current via line (12). The distance between the imaging lens and the object to be measured forms an optical detour (13) which is considerably lengthened compared to the reference arm.

PRIOR ART

The invention is based on a laser vibrometer for vibration measurementswhich operates according to the principle of the heterodyneinterferometer.

It is known that detailed vibration examinations are necessary,particularly in automobile manufacturing, to analyze resonance phenomenaon components. For example, in an article "Schwingungen visualisieren"Visualizing Vibrations! by Martin Feser in the ATZ AutomobiltechnischeZeitschrift Automotive Engineering Journal! 96 (1994) 7/8, page 433-435,a laser scanning vibrometer is described which determines the frequencyspectrum for scanned measuring points by way of non-contacting laserDoppler measuring technology and fast FET analysis. A core element ofthis known scanning vibrometer is a one-point vibrometer. In themeasuring head, the laser beam of an interferometer is guided with twoscanning mirrors onto the surface to be examined. In addition to thelaser interferometer and the scanning mirrors, the optical measuringhead also comprises a video camera with which the object to be measuredis observed. By way of the video image, the region to be scanned isdefined with local resolution whose setting can be varied and the regionis subsequently scanned by the laser beam. Because of the option of thefrequency band selection, it is possible to determine the modes of theoperating vibrations. The measuring system described here dealsexclusively with the application for the measurement of surfacevibrations, using in particular the example of automobile doors. Byusing a few defined frequencies, it is intended to determine points atwhich damping measures, necessary because of vibration amplitudes thatare too large, should be determined and introduced already during thedesign phase, if possible.

This known laser vibrometer of the company Polytec GmbH, D-76337Waldbronn, is described in detail in a manual VIB-MAN-9308-e04 inchapter 5 on pages 5-1 to 5-11, also with respect to the underlyingphysical measuring technology as well as the equipment configuration.For the necessary frequency shift, the laser beam is modulated by meansof an acoustooptical modulator in the form of a Bragg cell. For themodulation, the Bragg cell is supplied with high-frequency power. Thismodulation of the reference laser beam thus requires a rather costlyacoustooptical modulator which must be supplied with considerablehigh-frequency modulation power and, in addition, requires an extensiveadjustment.

In an article "Diode laser direct modulation heterodyne interferometer"by Kimio Tatsuno and Yoshito Tsunoda in APPLIED OPTICS, Vol. 26, No. 1,1 Jan. 1987, pages 37 to 40, an interferometer is provided for measuringthe wave front aberration of optical heads in systems with opticalplates, in which interferometer a frequency shift is generated bychanging the injection current of the laser diode.

SUMMARY AND ADVANTAGES OF THE INVENTION

According to the present invention there is provided a laser vibrometerfor vibration measurements which operates according to the principle ofthe heterodyne interferometer, and wherein in order to attain thenecessary frequency shift (Δv), a modulation of the frequency of thelaser radiation, preferably of a laser diode, occurs, and aninterferometer arm, preferably the interferometer arm in the beam pathto the object to be measured, is lengthened considerably compared to thereference arm, in particular, larger than 20 cm and up to approximately100 cm.

In contrast, to the prior art vibrometer discussed above the laservibrometer according to the invention having the features describedabove offers the advantage that the manner in which the necessaryfrequency shift is accomplished is much simpler. This manner is alsomuch more economical and less elaborate with respect to the adjustment.Therewith, the invention makes available an economical, non-contacting,optical vibrometer which can be used in many different ways.

This is accomplished according to the core concept of the invention inthat a modulation of the frequency of the laser radiation, preferably ofthe laser diode, occurs and an interferometer arm, preferably theinterferometer arm in the beam path to the object to be measured, islengthened considerably compared to the reference arm, in particular,larger than 20 cm and up to approximately 100 cm.

By way of further disclosed measures, advantageous modifications,improvements and advantageous applications of the above described laservibrometer according to the invention are possible.

According to a particularly advantageous embodiment of the invention,the modulation of the frequency of the laser radiation occurs throughmodulating the laser injection current, in particular, bytriangular-shaped or sawtooth-shaped modulation. Based on thismodulation, the laser vibrometer of the invention can be referred to asa directly modulated laser vibrometer.

According to a useful modification of the laser vibrometer according tothe invention, the triangular-shaped or sawtooth-shaped modulation takesplace with an amplitude j of the current modulation and a frequency f ofthe current modulation, wherein the amplitude can be in the range ofseveral mA and the frequency in the range from less than one up toseveral MHz.

A further advantageous feature of the laser vibrometer according to theinvention provides that a Michelson interferometer is used for thevibration measurement, in particular, the axial, so-called out of planevibration measurement.

According to a particularly advantageous feature of the laser vibrometerof the invention, a polarization beam splitter is provided having twoquarter-wave plates, one each in the reference beam path and one each inthe beam path of the object to be measured.

A first example of the particularly advantageous use of the laservibrometer designed according to the invention provides that it is usedfor the vibration measurement on devices or components thereof to detectpoints which cause great and/or unpleasant noise.

A second example of the particularly advantageous use of the laservibrometer designed according to the invention provides that it is usedfor the vibration measurement on devices or components thereof to detectpoints at which there exists risk of fracture and or the risk ofpremature wear due to high and/or, with respect to frequency,particularly critical vibration amplitudes.

A further example of the particularly advantageous use of the laservibrometer designed according to the invention provides that it is usedfor the vibration measurement on devices or components thereof to detectrhythmically recurring vibrations which, in terms of their evaluation,are convertible particularly into rotational speeds of internalcombustion engines.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in greater detail in the description below byway of an embodiment which is illustrated in the drawing, wherein:

FIG. 1 is a schematic representation of the essential components of thedirectly modulated laser vibrometer designed according to the invention;

FIG. 2 schematically shows the sawtooth-shaped modulated injectioncurrent j of the laser diode over the time t;

FIG. 3 schematically shows the frequency v, obtained on the basis of thedirect modulation, over the time t to attain the heterodyne frequencyΔv; and,

FIG. 4 is a diagram with an interference output signal in the uppersection and the associated laser diode injection current in the lowersection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In a schematic representation, FIG. 1 shows the essential components ofthe directly modulated laser vibrometer designed according to theinvention. A Michelson interferometer is used for the vibrationmeasurement, in particular, the axial, the so-called out of planevibration measurement. A laser diode 1 emits light which is guided in aparallel beam path onto a polarization beam splitter 3 by a lens 2. Aportion of the beam is guided through directly to an imaging lens 4 anda second portion of the beam is deflected by 45° onto a mirror 5.Quarter-wave plates 6 and 7 are arranged between the polarization beamsplitter 3 and respectively the lens 4 or the mirror 5. Here, thequarter-wave plate 7 has the effect that the beam which is reflected onthe mirror 5 is turned such that it is guided completely through thepolarization beam splitter 3 onto a photodetector unit 8. Here, thequarter-wave plate 6 has the effect that the beam, which is reflected byan object 9 to be measured, is deflected completely by 45° in thepolarization beam splitter 3 and is guided into the photodetector unit8.

The output signal of the photodetector unit 8, the interference signalgenerated from the superposition of the two reflected beams, is suppliedto an electronic evaluation equipment 10. The latter is also suppliedwith a signal of a current modulator 11 which, on the other hand,supplies the laser diode 1 with modulated injection current j via a line12 according to the invention. The distance between the imaging lens 4and the object 9 to be measured forms an optical detour 13 or an arm ofthe interferometer. According to the invention, this interferometer arm13 is considerably lengthened compared to the reference arm. The value,i.e., the length of this optical detour can be particularly larger than20 cm and amount to up to approximately 100 cm. The object 9 to bemeasured vibrates particularly in the direction of the double arrow 14.This direction of vibration along the beam path is identified as theaxial or out of plane direction.

As is shown in FIG. 2 by way of example, the injection current of thelaser diode 1 is modulated in sawtooth shape or triangular shape, namelywith the amplitude j and the frequency f. In FIG. 2, the amplitude j isplotted over the time t. As a result of a thermal effect, the opticallength of the resonator is changed through the change of the refractiveindex. This generates a modulation of the frequency of the laserradiation. The frequency-current coefficient β is different fordifferent laser diodes and is a function of temperature and modulationfrequency. It may, for example, have a size of approximately 3 GHz/mA.

As is shown in FIG. 1, the light beam of the laser diode 1 is divided inthe polarization beam splitter 3. The returning waves, which arereflected by the mirror 5 and the object 9 to be measured reach thephotodetector unit 8 at different times.

In FIG. 3, the frequency v is plotted over the time t so as toschematically illustrate the relationships during the superposition ofthe signals arriving at the photodetector unit 8. The two reflectedwaves are identified by 35 and 39 and reach the photodetector unit 8 atthe two different moments designated with A and B. The thus resultingtransit time difference At corresponds to the transit time of the laserlight beam through the considerably lengthened interferometer arm 13with the length ΔL. From the transit time difference At results acorresponding frequency difference Δv for the two reflected waves 35 and39. This frequency difference is also identified as the beat frequencyor heterodyne frequency and can be determined with the assistance of thefollowing equation:

    Δv=j*β*f*ΔL*C.sup.-1

FIG. 4 illustrates over the time axis t, indicated here in millisecondsms, in the upper section an example for a measured interference outputsignal, namely two light waves with different frequency, and in thelower section the associated sawtooth-shaped injection current.

By way of a numerical example and the equation indicated above, a valuefor the frequency shift is intended to be elucidated below:

    ______________________________________    j = 2 mA       amplitude of the current modulation    β = 3 * 10.sup.9 Hz/mA                   frequency-current coefficient of the                   laser diode    f = 10.sup.6 Hz                   frequency of the current modulation    ΔL = 2*50 cm - 1 m                   double the distance between laser                   vibrometer and object    Δv = 2*3*10.sup.9 *10.sup.6 /3*10.sup.8  Δv = 20    ______________________________________    Mz.

Therewith, the invention provides a laser vibrometer which accomplishesthe frequency shift required for recognizing the direction of the axialvibration of the object to be measured by simple means, direct currentmodulation of the laser diode and considerable lengthening of aninterferometer arm.

This laser vibrometer designed according to the invention can be usedwith great benefit in many different areas of application in aparticularly advantageous manner. Thus, according to a first example, itcan be used in an especially advantageous manner to determine points forthe vibration measurement on devices or components thereof which causegreat and/or unpleasant noise or at which there exists risk of fractureand/or the risk of premature wear due to high and/or, with respect tofrequency, particularly critical vibration amplitudes.

A further example of the especially advantageous use of the laservibrometer designed according to the invention provides that it is usedfor the vibration measurement on devices or components thereof to detectrhythmically recurring vibrations which, in terms of their evaluation,are convertible particularly into rotational speeds of internalcombustion engines.

We claim:
 1. A laser vibrometer for vibration measurements whichoperates according to the principle of the heterodyne interferometer,comprising in combination: means including a laser diode for producing aparallel beam of light radiation; a Michelson interferometer includingmeans for receiving the parallel beam and splitting same into firstpartial beam in a reference arm and a second partial beam in a measuringarm, with the first partial beam being reflected from a mirror disposedat a fixed position and the second partial beam being reflected from anobject to be measured disposed at a known distance, and for combiningthe reflected partial beams to form an interference pattern in aninterference arm, with one of the reference and measuring arms beingconsiderably longer than the other by approximately 20 cm toapproximately 100 cm; means for modulating the frequency of the laserproduced radiation to attain a necessary frequency shift (Δv); a singledetector for detecting the interference pattern in said interferencearm; and an evaluation circuit means, responsive to the detectedinterference pattern and an output signal from said means for modulatingrepresenting the modulation, for providing an output signalcorresponding to the axial out of plane vibration of the test object. 2.A laser vibrometer according to claim 1, wherein the means formodulating modulates the frequency of the laser radiation throughmodulating the laser injection current.
 3. A laser vibrometer accordingto claim 2, wherein the means for modulating modulates the laserinjection current with a triangular-shaped or sawtooth-shapedmodulation.
 4. A laser vibrometer according to claim 3, wherein thetriangular-shaped or sawtooth-shaped modulation takes place with anamplitude j of the current modulation and a frequency f of the currentmodulation.
 5. A laser vibrometer according to claim 4, wherein theamplitude of the modulation is in the range of 2 to 15 mA and thefrequency of the modulation is in the range from less than one up toseveral MHZ.
 6. A laser vibrometer according to claim 1, wherein themeans for splitting includes a polarization beam splitter provided withtwo quarter-wave plates, one in the reference beam path and one in thebeam path of the object to be measured.
 7. A laser vibrometer accordingto claim 1, wherein the measuring arm is the longer arm.
 8. A method ofmeasuring the vibration of devices or components thereof to detectpoints which cause great and/or unpleasant noise comprising the stepof:providing a laser vibrometer which operates according to theprinciple of the heterodyne interferometer, and which comprises meansincluding a laser diode for producing a parallel beam of lightradiation; a Michelson interferometer including means for receiving theparallel beam and splitting same into first partial beam in a referencearm and a second partial beam in a measuring arm, with the first partialbeam being reflected from a mirror disposed at a fixed position and thesecond partial beam being reflected from an object to be measureddisposed at a known distance, and for combining the reflected partialbeams to form an interference pattern in an interference arm, with oneof the reference and measuring arms being considerably longer than theother by approximately 20 cm to approximately 100 cm; means formodulating the frequency of the laser produced radiation to attain anecessary frequency shift (Δv); a single detector for detecting theinterference pattern in the interference arm; and an evaluation circuitmeans, responsive to the detected interference pattern and an outputsignal from said means for modulating representing the modulation, forproviding an output signal corresponding to the axial out of planevibration of the test object; positioning the vibrometer such that thedevice constitutes the object to be measured; and, measuring the axialout of plane vibration of the device with the vibrometer.
 9. A method ofmeasuring the vibration of devices or components thereof to detectpoints at which there exists risk of fracture and/or the risk ofpremature wear due to high and/or, with respect to frequency,particularly critical vibration amplitudes; comprising the stepsof:providing a laser vibrometer which operates according to theprinciple of the heterodyne interferometer, and which comprises meansincluding a laser diode for producing a parallel beam of lightradiation; a Michelson interferometer including means for receiving theparallel beam and splitting same into first partial beam in a referencearm and a second partial beam in a measuring arm, with the first partialbeam being reflected from a mirror disposed at a fixed position and thesecond partial beam being reflected from an object to be measureddisposed at a known distance, and for combining the reflected partialbeams to form an interference pattern in an interference arm, with oneof the reference and measuring arms being considerably longer than theother by approximately 20 cm to approximately 100 cm; means formodulating the frequency of the laser produced radiation to attain anecessary frequency shift (Δv); a single detector for detecting theinterference pattern in the interference arm; and an evaluation circuitmeans, responsive to the detected interference pattern and an outputsignal from said means for modulating representing the modulation, forproviding an output signal corresponding to the axial out of planevibration of the test object; positioning the vibrometer such that thedevice constitutes the object to be measured; and, measuring the axialout of plane vibrations of the device with the vibrometer.
 10. A amethod of measuring the vibration of devices or components thereof todetect rhythmically recurring vibrations which, in terms of theirevaluation, are convertible into rotational speeds of internalcombustion engines comprising the steps of:providing a laser vibrometerwhich operates according to the principle of the heterodyneinterferometer, and which comprises means including a laser diode forproducing a parallel beam of light radiation; a Michelson interferometerincluding means for receiving the parallel beam and splitting same intofirst partial beam in a reference arm and a second partial beam in ameasuring arm, with the first partial beam being reflected from a mirrordisposed at a fixed position and the second partial beam being reflectedfrom an object to be measured disposed at a known distance, and forcombining the reflected partial beams to form an interference pattern inan interference arm, with one of the reference and measuring arms beingconsiderably longer than the other by approximately 20 cm toapproximately 100 cm; means for modulating the frequency of the laserproduced radiation to attain a necessary frequency shift (Δv); a singledetector for detecting the interference pattern in the interference arm;and an evaluation circuit means, responsive to the detected interferencepattern and an output signal from said means for modulating representingthe modulation, for providing an output signal corresponding to theaxial out of plane vibration of the test object; positioning thevibrometer such that the device constitutes the object to be measured;and, measuring the axial out of plane vibrations of the device with thevibrometer.